The presence of large deposits of oil shale in the Rocky Mountain region of the United States has given rise to extensive efforts to develop methods of recovering shale oil from kerogen in the oil shale deposits. It should be noted that the term "oil shale" as used in the industry is in fact a misnomer; it is neither shale nor does it contain oil. It is a sedimentary formation comprising marlstone deposit interspersed with layers containing an organic polymer called "kerogen", which upon heating decomposes to produce carbonaceous liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein, and the liquid carbonaceous product is called "shale oil".
A number of methods have been developed for processing the oil shale which involve either first mining the kerogen bearing shale and processing the shale above ground, or processing the shale in situ. The latter approach is preferable from the standpoint of environmental impact since the spent shale remains in place, reducing the chance of surface contamination and the requirement for disposal of solid wastes.
The recovery of liquid and gaseous products from oil shale deposits has been described in several patents, one of which is U.S. Pat. No. 3,661,423, issued May 9, 1972 to Donald E. Garrett, assigned to the assignee of this application, and incorporated herein by reference. This patent describes in situ recovery of liquid and gaseous carbonaceous materials from a subterranean oil shale deposit by mining out a portion of shale in a subterranean oil shale deposit and then fragmenting and expanding a portion of the remaining oil shale in the deposit to form a stationary body of fragmented oil shale within the deposit, referred to herein as an in situ oil shale retort. Hot retorting gases are passed through the in situ oil shale retort to convert kerogen contained in the oil shale to liquid and gaseous products.
One method of supplying hot retorting gases used for converting kerogen contained in the oil shale, as described in U.S. Pat. No. 3,661,423, includes establishment of a combustion zone in the retort and introduction of an oxygen supplying gaseous feed mixture downwardly into the combustion zone to advance the combustion zone downwardly through the retort. In the combustion zone oxygen in the gaseous feed mixture is depleted by reaction with hot carbonaceous materials to produce heat and combustion gas. By the continued introduction of the oxygen supplying gaseous feed mixture downwardly into the combustion zone, the combustion zone is advanced downwardly through the retort.
The effluent gas from the combustion zone comprises combustion gas and the portion of the gaseous feed mixture which does not take part in the combustion process. This effluent gas is essentially free of free oxygen and contains constituents such as oxides of carbon and sulfurous compounds. It passes through the retort on the advancing side of the combustion zone to heat the oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called retorting, in the oil shale to gaseous and liquid products and a residue product of solid carbonaceous material.
The liquid products and gaseous products are cooled by the cooler oil shale fragments in the retort on the advancing side of the retorting zone. The liquid carbonaceous products, together with water produced in or added to the retort, are collected at the bottom of the retort and withdrawn to the surface through an access tunnel, drift or shaft. An off gas containing combustion gas generated in the combustion zone, product gas produced in the retorting zone, gas from carbonate decomposition, and gaseous feed mixture which does not take part in the combustion process is also collected at the bottom of the retort and withdrawn to the surface. The off gas contains constituents such as hydrogen, carbon monoxide, carbon dioxide, hydrocarbons, and hydrogen sulfide.
In introducing oxygen supplying gas into the retort, the pressure within the retort can rise to a level greater than the pressure in adjacent access tunnels in the formation. As a result, some leakage of gases from the retort can take place through cracks and faults in the formation into the tunnels. Since gases downstream of the combustion zone contain noxious sulfurous compounds and carbon monoxide which is poisonous, leakage of these gases from the retort can give rise to dangerous conditions in tunnels adjacent the retort.
Therefore, there is a need for a process and apparatus for retorting oil shale in situ which prevents leakage of gas from a retort into tunnels adjacent the retort.